DOI: 10.1002/adma.200702451 Towards Solutions of Single-Walled Carbon Nanotubes in Common Solvents** By Shane D. Bergin, Valeria Nicolosi, Philip V. Streich, Silvia Giordani, Zhenyu Sun, Alan H. Windle, Peter Ryan, N. Peter P. Niraj, Zhi-Tao T. Wang, Leslie Carpenter, Werner J. Blau, John J. Boland, James P. Hamilton, * and Jonathan N. Coleman* The single most common processing technique for carbon nanotubes involves dispersing them in liquid media. [1] The resulting suspensions can then be used for fundamental studies of the properties of the nanotubes themselves [2] or can be further processed into useful structures such as sheets [3] or fibers. [4] This procedure has been incredibly successful in spite of one huge obstacle: owing to their high molecular weight, nanotubes are considered insoluble in all known solvents. This handicap has generally been circumvented by coating the nanotubes with a dispersant phase, usually a surfactant, [5] a biomolecule, [6] or a polymer, [7] resulting in weak internano- tube repulsions and a metastable suspension of the coated nanotubes in the solvent of choice. Alternatively, nanotubes can be dispersed subsequent to chemical treatment such as functionalization, [8–10] reduction by alkali metals, [11] or by protonation with superacids; [12] all techniques that tend to alter nanotube properties such as electronic structure. However, a significant improvement would be to disperse pristine nanotubes in common solvents without the aid of a third, dispersant phase. In fact, the ideal situation would be to find a solvent in which nanotubes were thermodynamically soluble, that is, where the free-energy of mixing, DG Mix , is negative. This would allow established techniques for processing soluble polymers to be brought to bear. In addition, it would facilitate exfoliation of nanotubes from bundles, a significant problem. The free energy of mixing is given by DG Mix ¼ DH Mix  TDS Mix , where DH Mix and DS Mix are the enthalpy and entropy of mixing, respectively. [13] For most molecular combinations, DH Mix is small and positive and so dissolution is usually driven by a very large value of DS Mix . However, the large molecular weight and high rigidity of nanotubes leads to an extremely small entropy of mixing. Due to their large mutual attraction, DH Mix is generally expected to be positive for all conceivable polymer–nanotube mixtures, resulting in a positive DG Mix , prohibiting nanotube dissolution. In this work, we unambiguously demonstrate that, for the case of the solvent N-methyl-pyrrolidone (NMP), the single-walled carbon nano- tube (SWNT)–NMP interaction leads to an enthalpy of mixing that is approximately zero, and hence a negative free energy of mixing. We demonstrate that nanotubes spontaneously exfoliate on dilution of SWNT–NMP dispersions, leading to a dynamic equilibrium characterized by significant populations of pristine individual nanotubes and small bundles. A detailed thermodynamic analysis of the SWNT–NMP system is presented and provides a template to identify and/or develop other possible solvent systems. It is known that SWNTs can be dispersed [14,15] and even exfoliated by sonication and dilution in NMP. [16] Here we suggest that these dispersions are actually solutions in the true thermodynamic sense. We begin by characterizing SWNT– NMP dispersions prepared at a range of concentrations by successive dilution and sonication steps. [16] By casting onto SiO 2 substates and characterizing by atomic force microscopy (AFM) we can determine the root-mean-square bundle diameter, as shown in Figure 1A. The bundle diameter tends to decrease with decreasing nanotube concentration and this can be modeled under the assumption that a concentration- independent equilibrium number density of bundles exists, COMMUNICATION [*] Prof. J. P. Hamilton, P. V. Streich Department of Chemistry and Engineering Physics University of Wisconsin–Platteville Platteville, WI 53818 (USA) E-mail: hamiltoj@uwplatt.edu Prof. W. J. Blau, Dr. S. Giordani School of Physics, Trinity College Dublin University of Dublin Dublin 2 (Ireland) E-mail: colemaj@tcd.ie Prof. J. N. Coleman, Dr. S. D. Bergin, Dr. V. Nicolosi, Dr. Z. Sun Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN) School of Physics, Trinity College Dublin University of Dublin Dublin 2 (Ireland) Prof. A. H. Windle Department of Materials Science and Metallurgy University of Cambridge Pembroke Street, Cambridge, CB2 3QZ (UK) Prof. J. J. Boland, Dr. Z. T. Wang Dr. S. Giordani, N. P. P. Niraj P. Ryan Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN) School of Chemistry, Trinity College Dublin University of Dublin Dublin 2 (Ireland) Dr. L. Carpenter Dow Corning Corporation 3901 S. Saginaw Rd. Midland, MI 48640, (USA) [**] Supporting Information is available online from Wiley InterScience or from the authors. 1876 ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2008, 20, 1876–1881